Rubber composition

ABSTRACT

The present invention provides a rubber composition containing: (A) a rubber component containing 10% by mass or more of at least one kind of rubber selected from diene rubber synthesized by emulsion polymerization and natural rubber and 90% by mass or less of another kind of diene rubber; (B) silica having a n-hexadecyltrimethylammonium bromide (CTAB) adsorption specific surface area of not less than 180 m 2 /g measured according to a method described in ASTM D3765-92; (C) at least one silane coupling agent selected from a polysulfide compound and a thioester compound; and (D) a vulcanization accelerator, the rubber composition after vulcanization having an average aggregated aggregate area (nm 2 ) of the silica of 1,900 or less, and thus provides a rubber composition that is improved in low-heat-generation property.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2012/061491, filed on Apr. 27, 2012, which claims priority fromJapanese Patent Application No. 2011-102335, filed on Apr. 28, 2011, thecontents of all of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a rubber composition containing silica,which has an improved low-heat-generation property.

BACKGROUND ART

Recently, in association with the movement of global regulation ofcarbon dioxide emission associated with the increase in attraction toenvironmental concerns, the demand for low fuel consumption byautomobiles is increasing. To satisfy the requirement, it is desired toreduce rolling resistance relating to tire performance. Heretofore, as ameans for reducing the rolling resistance of tires, a method ofoptimizing tire structures has been investigated; however, at present, atechnique of using a low-heat-generating rubber composition for tireshas been employed as the most common method.

For obtaining such a low-heat-generating rubber composition, there isknown a method of using an inorganic filler such as silica or the like.

However, in the rubber composition containing silica compounded therein,silica aggregates (owing to the hydroxyl group in the surface ofsilica), and therefore, for preventing the aggregation, a silanecoupling agent is used.

Accordingly, for successfully solving the above-mentioned problem byincorporation of a silane coupling agent, various trials have been madefor increasing the activity of the coupling function of the silanecoupling agent.

For example, PTL 1 proposes a rubber composition containing, as basiccomponents, at least (i) one diene elastomer, (ii) a white filler as areinforcing filler and (iii) an alkoxysilane polysulfide as a couplingagent (white filler/diene elastomer) along with (iv) an enamine and (v)a guanidine derivative.

PTL 2 discloses a rubber composition containing, as basic components, atleast (i) one diene elastomer, (ii) a white filler as a reinforcingfiller and (iii) an alkoxysilane polysulfide as a coupling agent (whitefiller/diene elastomer) along with (iv) zinc dithiophosphate and (v) aguanidine derivative.

PTL 3 describes a rubber composition containing, as basic components, atleast (i) a diene elastomer, (ii) an inorganic filler as a reinforcingfiller and (iii) an alkoxysilane polysulfide (PSAS) as an (inorganicfiller/diene elastomer) coupling agent, as combined with (iv) analdimine (R—CH═N—R) and (v) a guanidine derivative.

Further, PTL 4 proposes a rubber composition basically containing atleast (i) a diene elastomer, (ii) an inorganic filer as a reinforcingfiler and (iii) an alkoxysilane polysulfide as a coupling agent, ascombined with (iv) 1,2-dihydropyridine and (v) a guanidine derivative.PTL 5 proposes a technique of increasing the activity of the couplingfunction of a silane coupling agent in consideration of kneadingconditions.

PTL 6 describes an invention, in which silica having an average particlediameter of 10 μm or less and a specific silane coupling agent are addedto a rubber composition, thereby suppressing aggregation of the silica.

PTL 7 proposes a technique, in which silica that preferably has an-hexadecyltrimethylammonium bromide (CTAB) adsorption specific surfacearea of from 60 to 250×10² m²/kg and an tea extract containing catechinare added to a rubber composition, thereby preventing large aggregatesof the silica from being present in the rubber composition.

Furthermore, PTL 8 and PTL 9 describe a rubber composition that has sucha dispersion state of the filler contained in the rubber component thatthe area ratio occupied by filler aggregates having a circle-equivalentdiameter of 10 μm or more based on the total observed area in adispersion evaluation method of observing a cut surface of a specimen bya dark field method is 2.0% or less.

However, there is a demand of a technique for further enhancing thelow-heat-generation property of a rubber composition containing silica.

CITATION LIST Patent Literatures

-   PTL 1: JP-T 2002-521515-   PTL 2: JP-T 2002-521516-   PTL 3: JP-T 2003-530443-   PTL 4: JP-T 2003-523472-   PTL 5: WO2008/123306-   PTL 6: JP-A 2009-256576-   PTL 7: JP-A 2010-031260-   PTL 8: JP-A 2010-248422-   PTL 9: JP-A 2010-248423

SUMMARY OF INVENTION Technical Problem

Under the circumstances, an object of the present invention is toprovide a rubber composition that is improved in low-heat-generationproperty.

Solution to Problem

The present inventors have paid attention to the dispersion state ofsilica in a rubber composition, and tried to evaluate the dispersionstate of silica by various measurement methods. As a result, it has beenfound that the low-heat-generation property may be enhanced throughreduction of the hysteresis properties (particularly, tan δ) of therubber composition by making an average aggregated aggregate areaaccording to a particular measurement method to be a particular value orless, and thus the present invention has been completed.

The present invention thus relates to a rubber composition containing:(A) a rubber component containing 10% by mass or more of at least onekind of rubber selected from diene rubber synthesized by emulsionpolymerization and natural rubber and 90% by mass or less of anotherkind of diene rubber; (B) a silica having a n-hexadecyltrimethylammoniumbromide (CTAB) adsorption specific surface area of not less than 180m²/g measured according to a method described in ASTM D3765-92; (C) atleast one silane coupling agent selected from a polysulfide compound anda thioester compound; and (D) a vulcanization accelerator, the rubbercomposition after vulcanization having an average aggregated aggregatearea (nm²) of the silica of 1,900 or less, measurement method of averageaggregated aggregate area:

-   -   an upper surface of a specimen of the rubber composition after        vulcanization is cut in a direction making an angle of 38° with        respect to the upper surface of the specimen with a focused ion        beam; then a smooth surface of the specimen formed by cutting is        imaged with a scanning electron microscope at an acceleration        voltage of 5 kV in a direction perpendicular to the smooth        surface; the resulting image is converted to a binarized image        of a rubber portion and a silica portion as a filler of the        specimen by the Otsu's method; an aggregated aggregate area of        the silica portion is obtained based on the resulting binarized        image; and the average aggregated aggregate area of the silica        portion is calculated in terms of number average (arithmetic        average) per unit area (3 μm×3 μm) from a total surface area of        the silica portion and the number of aggregated aggregates,        provided that in the calculation, a particle that is in contact        with an edge of the image is not counted, and a particle of 20        pixels or less is assumed to be noise and is not counted.

Advantageous Effects of Invention

According to the present invention, a rubber composition that isimproved in low-heat-generation property may be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an image showing an example of an FIB-SEM image obtained byimaging aggregated aggregates of silica in the rubber composition of thepresent invention by the measurement method of an average aggregatedaggregate area according to the present invention.

FIG. 2 is an image showing an example of a binarized image of the imageshown in FIG. 1.

FIG. 3 is an image showing a reference example of an FIB-SEM imageobtained by imaging aggregated aggregates of silica by the same methodas in FIG. 1.

FIG. 4 is an image showing an example of a binarized image of the imageshown in FIG. 3.

DESCRIPTION OF EMBODIMENTS

The present invention will be described in detail below.

The rubber composition of the present invention contains: (A) a rubbercomponent containing 10% by mass or more of at least one kind of rubberselected from diene rubber synthesized by emulsion polymerization andnatural rubber and 90% by mass or less of another kind of diene rubber;(B) a silica having a n-hexadecyltrimethylammonium bromide (CTAB)adsorption specific surface area of not less than 180 m²/g measuredaccording to a method described in ASTM D3765-92; (C) at least onesilane coupling agent selected from a polysulfide compound and athioester compound; and (D) a vulcanization accelerator, and the rubbercomposition after vulcanization has an average aggregated aggregate area(nm²) of the silica of 1,900 or less. For further enhancing thelow-heat-generation property of the rubber composition, the averageaggregated aggregate area (nm²) of the silica is preferably 1,800 orless, and more preferably 1,700 or less. The average aggregatedaggregate area (nm²) of the silica is preferably 300 or more, morepreferably from 300 to 1,900, further preferably from 300 to 1,800, andparticularly preferably from 300 to 1,700.

The measurement method of the average aggregated aggregate area is asfollows. An upper surface of a specimen of the rubber composition aftervulcanization is cut in a direction making an angle of 38° with respectto the upper surface of the specimen with a focused ion beam, and then asmooth surface of the specimen formed by cutting is imaged with ascanning electron microscope at an acceleration voltage of 5 kV in adirection perpendicular to the smooth surface. The resulting image isconverted to a binarized image of a rubber portion and a silica portionas a filler of the specimen by the Otsu's method, an aggregatedaggregate area of the silica portion is obtained based on the resultingbinarized image, and the average aggregated aggregate area is calculatedin terms of number average (arithmetic average) per unit area (3 μm×3μm) from the total surface area of the silica portion and the number ofaggregated aggregates. In the calculation, a particle that is in contactwith an edge of the image is not counted, and a particle of 20 pixels orless is assumed to be noise and is not counted.

In the measurement of the average aggregated aggregate area in thepresent invention, FIB-SEM, which is an integrated equipment of afocused ion beam machining observation device (FIB) and a scanningelectron microscope (SEM), is preferably used. The scanning electronmicroscope (SEM) used is preferably an ultra low acceleration voltagescanning electron microscope.

Examples of the FIB-SEM include “NOVA 200”, a trade name (registeredtrademark), produced by FEI Company, and “SMI-3050MS2”, a trade name(registered trademark), produced by SII Nano Technology Inc., and “NOVA200”, a trade name (registered trademark), produced by FEI Company, ispreferably used.

For converting to a binarized image, an image processing device by theOtsu's method may be used.

In the measurement of the average aggregated aggregate area in thepresent invention, the upper surface of the specimen of the rubbercomposition after vulcanization is cut in a direction making an angle of38° with respect to the upper surface of the specimen with a focused ionbeam, and then a smooth surface of the specimen formed by cutting isimaged with a scanning electron microscope at an acceleration voltage of5 kV in a direction perpendicular to the smooth surface. In this method,a high precision image of the smooth cross sectional surface of thespecimen containing only the surface information of the cross sectionalsurface can be obtained without influence of fluctuation in brightness,out of focus and the like, as seen in the related art. Accordingly, thedispersion state of the filler in the polymer material may bedigitalized based on the resulting high precision image, and the averageaggregated aggregate area of the rubber composition after vulcanizationcontaining silica may be quantitatively evaluated. In the case where aspecimen is cut with FIB, the cut surface that is formed in a directionparallel to the radiation direction of FIB becomes a smooth surface, andthe cut surface that is formed in a direction perpendicular to theradiation direction of FIB becomes a rough surface having unevenness.Accordingly, the smooth surface to be imaged in the present inventionmeans the cut surface that is formed in a direction parallel to theradiation direction of FIB.

Subsequently, the threshold value for binarization of the resultingimage is determined by the Otsu's method. The resulting image isconverted to a binarized image of the rubber portion and the silicaportion as a filler of the specimen with the threshold value, anaggregated aggregate area of the silica portion is obtained based on theresulting binarized image, and the average aggregated aggregate area iscalculated in terms of number average (arithmetic average) per unit area(3 μm×3 μm) from the total surface area of the silica portion and thenumber of aggregated aggregates. In the calculation, a particle that isin contact with an edge of the image is not counted, and a particle of20 pixels or less is assumed to be noise and is not counted.

FIG. 1 is an image showing an example of an FIB-SEM image obtained byimaging aggregated aggregates of silica in the rubber composition of thepresent invention by the measurement method of an average aggregatedaggregate area according to the present invention, and FIG. 2 is animage showing an example of a binarized image of the image shown in FIG.1.

FIG. 3 is an image showing a reference example of an FIB-SEM imageobtained by imaging aggregated aggregates of silica by the same methodas in FIG. 1, and FIG. 4 is an image showing an example of a binarizedimage of the image shown in FIG. 3.

The aggregated aggregate in the present invention means an agglomeratedmatter of plural aggregates, and encompasses a single aggregate. Theaggregate (i.e., a primary aggregated matter) herein means a complexaggregated form of silica formed by fusing primary particles of silicato form linear or irregularly branched chains, and may have a size offrom several ten to several hundred nanometers.

The aggregated aggregate in the present invention is far smaller than anagglomerate (i.e., a secondary aggregated matter), which is consideredto have, in general, a size of from several ten to several hundredmicrometers, and these are concepts that are completely different fromeach other.

The n-hexadecyltrimethylammonium bromide (CTAB) adsorption specificsurface area (which may be hereinafter abbreviated as a “CTAB adsorptionspecific surface area”) of silica is measured according to a methoddescribed in ASTM D3765-92, as described above. However, smallmodifications are made in the method since the method described in ASTMD3765-92 is a method of measuring a CTAB adsorption specific surfacearea of carbon black. Specifically, IRB #3 (83.0 m²/g) as the standardproduct of carbon black is not used, but a n-hexadecyltrimethylammoniumbromide (CTAB) standard solution is separately prepared, with which anAerosol OT (sodium di-2-ethylhexyl sulfosuccinate) solution iscalibrated, and the specific surface area (m²/g) is calculated from theadsorption amount of CTAB assuming that the adsorption cross section perone molecule of CTAB on the surface of hydrated silicic acid is 0.35nm². The modifications are made because it is considered that carbonblack and hydrated silicic acid have surfaces that are different fromeach other, and thus there is a difference therebetween in the CTABadsorption amount on the same surface area.

[Rubber Component (A)]

The rubber component (A) used in the rubber composition of the presentinvention contains 10% by mass or more of at least one kind of rubberselected from diene rubber synthesized by emulsion polymerization andnatural rubber and 90% by mass or less of another kind of diene rubber,and preferably contains more than 10% by mass of at least one kind ofrubber selected from diene rubber synthesized by emulsion polymerizationand natural rubber and less than 90% by mass of another kind of dienerubber.

The diene rubber synthesized by emulsion polymerization in the presentinvention may be synthesized by an ordinary emulsion polymerizationmethod. Examples of the method include such a method that a prescribedamount of the monomer described later is emulsified in an aqueous mediumin the presence of an emulsifier and emulsion-polymerized with a radicalpolymerization initiator.

Examples of the emulsifier used include a long chain fatty acid salthaving 10 or more carbon atoms and/or a resin acid salt. Specificexamples thereof include potassium salts and sodium salts of capricacid, lauric acid, myristic acid, palmitic acid, oleic acid and stearicacid.

Examples of the radical polymerization initiator used include apersulfate salt, such as ammonium persulfate and potassium persulfate;and a redox initiator, such as a combination of ammonium persulfate andferric sulfide, a combination of an organic peroxide and ferric sulfide,and a combination of hydrogen peroxide and ferric sulfide.

For controlling the molecular weight of the diene rubber, a chaintransfer agent may be added. Examples of the chain transfer agent usedinclude a mercaptan compound, such as t-dodecylmercaptan andn-dodecylmercaptan, an α-methylstyrene dimer, carbon tetrachloride,thioglycolic acid, a diterpene compound, terpinolene and a γ-terpinenecompound.

The temperature of the emulsion polymerization may be appropriatelyselected depending on the kind of the radical polymerization initiatorused, and is generally from 0 to 100° C., and preferably from 0 to 60°C. The polymerization mode may be anyone of continuous polymerization,batch polymerization and the like.

The tendency of gelation may be observed when the polymerizationconversion is large in the emulsion polymerization. Accordingly, thepolymerization conversion is preferably suppressed to 90% or less, andin particular, the polymerization is preferably terminated at aconversion within a range of from 50 to 80%. The polymerization may beterminated generally by adding a polymerization terminator to thepolymerization system at the time when the prescribed conversion isachieved. Examples of the polymerization terminator used include anamine compound, such as diethyhydroxylamine and hydroxylamine, a quinonecompound, such as hydroquinone and benzoquinone, sodium nitrite, sodiumdithiocarbamate.

After terminating the polymerization reaction, the unreacted monomer isremoved depending on necessity from the resulting polymer latex, andthen after controlling the pH of the latex to a prescribed value byadding an acid, such as nitric acid and sulfuric acid, a salt, such assodium chloride, calcium chloride and potassium chloride, as acoagulating agent is added and mixed for coagulating the polymer into acram, which is then recovered. The cram is rinsed and dehydrated, andthen dried with a handy drier or the like to provide the target dienerubber.

Examples of a conjugated diene as a monomer used in the diene rubbersynthesized by emulsion polymerization in the present invention include1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene,2-chloro-1,3-butadiene and 1,3-pentadiene. Among these, 1,3-butadiene,2-methyl-1,3-butadiene and the like are preferred, and 1,3-butadiene ismore preferred. The conjugated diene may be used solely or as acombination of two or more kinds thereof. Examples of the aromatic vinylcompound include styrene, α-methylstyrene, 2-methylstyrene,3-methylstyrene, 4-methylstyrene, 2,4-diisopropylstyrene,2,4-dimethylstyrene, 4-tert-butylstyrene and5-tert-butyl-2-methylstyrene. Among these, styrene is preferred. Thearomatic vinyl compound may be used solely or as a combination of two ormore kinds thereof.

The diene rubber synthesized by emulsion polymerization in the presentinvention is preferably styrene-butadiene copolymer rubber (which may behereinafter referred to as “emulsion-polymerized SBR”).

The emulsion-polymerized SBR preferably contains a styrene component inan amount in a range of from 5 to 50% by mass, more preferably in arange of from 10 to 50% by mass, and further preferably in a range offrom 15 to 45% by mass.

The another kind of diene rubber in the rubber component (A) used in therubber composition of the present invention is preferably at least onekind of rubber selected from solution-polymerized styrene-butadienecopolymer rubber (which may be hereinafter referred to as“solution-polymerized SBR”), polybutadiene rubber (which may behereinafter referred to as “BR”) and synthesized polyisoprene rubber(which may be hereinafter referred to as “IR”). The solution-polymerizedSBR is preferably unmodified styrene-butadiene copolymer rubber (whichmay be hereinafter referred to as “unmodified solution-polymerized SBR”)and/or modified styrene-butadiene copolymer rubber having molecularchain ends modified with a tin compound (which may be hereinafterreferred to as “tin-modified solution-polymerized SBR”).

The another kind of diene rubber may be used solely or as a combinationof two or more kinds thereof.

The unmodified solution-polymerized SBR may be obtained by anionicpolymerization or coordination polymerization, and is preferablyproduced by anionic polymerization.

The polymerization initiator used in the anionic polymerization may bean alkali metal compound, and preferably a lithium compound. The lithiumcompound used may be not only an ordinary lithium compound (such as ahydrocarbyllithium and a lithiumamide compound), but also a lithiumcompound having a tin atom (such as a triorganotinlithium compound,e.g., tributyltinlithium and trioctyltinlithium), which may be used forproviding the tin-modified solution-polymerized SBR.

The tin-modified solution-polymerized SBR may be obtained in such amanner that after completing the polymerization reaction of unmodifiedsolution-polymerized SBR obtained above, a tin compound as a modifier isreacted with the polymerization active end of the styrene-butadienecopolymer before terminating the polymerization.

Examples of the tin compound include tin tetrachloride, tributyltinchloride, trioctyltin chloride, dioctyltin dichloride, dibutyltindichloride and triphenyltin chloride.

[Silica (B)]

The silica (B) used in the rubber composition of the present inventionmay be any one of commercially available products, and among these, wetmethod silica, dry method silica and colloidal silica are preferablyused, with wet method silica being more preferably used. Wet methodsilica is classified into precipitation method silica and gel methodsilica, and precipitation method silica is particularly preferred sinceit is easily dispersed in the rubber composition under shearing onkneading and is excellent in reinforcing effect due to the surfacereaction after dispersion.

It is characteristic that the silica (B) has a CTAB adsorption specificsurface area of not less than 180 m²/g, with from 180 to 300 m²/g beingpreferred. When the CTAB adsorption specific surface area is not morethan 300 m²/g, the processability of the unvulcanized rubber compositioncan be improved.

Preferred examples of the precipitation method silica that has a CTABadsorption specific surface area within the range include “Zeosil HRS1200”, a trade name (registered trademark), produced by Rhodia, Inc.(CTAB adsorption specific surface area: 200 m²/g).

The rubber composition of the present invention may contain carbon blackdepending on necessity in addition to the silica (B). The use of carbonblack contained provides such an effect that the electric resistance islowered to prevent static charge. The carbon black is not particularlylimited, and examples thereof include high, medium or low structurecarbon black, such as SAF, ISAF, IISAF, N339, HAF, FEF, GPF and SRFgrades, with carbon black of SAF, ISAF, IISAF, N339, HAF and FEF gradesbeing particularly preferably used. The carbon black used preferably hasa nitrogen adsorption specific surface area (N₂SA measured according toJIS K6217-1 (2001)) of from 30 to 250 m²/g. The carbon black may be usedsolely or as a combination of two or more kinds thereof.

The rubber composition of the present invention preferably contains thesilica (B) in an amount of from 25 to 150 parts by mass per 100 parts bymass of the rubber component (A). The amount of 25 parts by mass or moreis preferred from the standpoint of securing the wet capability, and theamount of 150 parts by mass or less is preferred from the standpoint ofdecreasing the rolling resistance. The silica (B) is more preferablycontained in an amount of from 25 to 120 parts by mass, and furtherpreferably in an amount of 30 to 85 parts by mass.

The rubber composition of the present invention preferably contains afiller, such as the silica (B), and carbon black that is added dependingon necessity in addition to the silica (B), in an amount of from 25 to170 parts by mass per 100 parts by mass of the rubber component (A). Theamount of 25 parts by mass or more is preferred from the standpoint ofenhancing the reinforcing property of the rubber composition, and theamount of 170 parts by mass or less is preferred from the standpoint ofdecreasing the rolling resistance.

The amount of the silica (B) in the filler is preferably 40% by mass ormore, and more preferably 70% by mass or more, for achieving both thewet capability and the rolling resistance.

[Silane Coupling Agent (C)]

The silane coupling agent (C) used in the rubber composition of thepresent invention is necessarily at least one silane coupling agent thatis selected from a polysulfide compound and a thioester compound. Thepolysulfide compound and the thioester compound are preferred since theyprevent scorch from occurring during kneading, thereby enhancing theprocessability.

The at least one silane coupling agent (C) selected from a polysulfidecompound and a thioester compound is preferably at least one compoundselected from the compounds represented by the following generalformulae (I) to (IV).

By using the silane coupling agent (C), the rubber composition accordingto the method of the present invention is further improved in theworkability on processing rubber and may provide a pneumatic tireexcellent in wear resistance.

Preferred examples of the polysulfide compound include the compoundsrepresented by the general formula (I) or (III), and preferred examplesof the thioester compound include the compounds represented by thegeneral formula (II) or (IV).

The general formulae (I) to (IV) are sequentially described below.[Chem. 1](R¹O)_(3-p)(R²)_(p)Si—R³—S_(a)—R³—Si(OR¹)_(3-r)(R²)_(r)  (I)wherein R¹, which may be the same or different, each represents alinear, cyclic or branched alkyl group, having from 1 to 8 carbon atoms,or a linear or branched alkoxylalkyl group, having from 2 to 8 carbonatoms; R², which may be the same or different, each represents a linear,cyclic or branched alkyl group, having from 1 to 8 carbon atoms; R³,which may be the same or different, each represents a linear or branchedalkylene group, having from 1 to 8 carbon atoms; a indicates from 2 to 6as a mean value; p and r, which may be the same or different, eachindicates from 0 to 3 as a mean value, provided that both p and r arenot 3 at the same time.

Specific examples of the silane coupling agent (C) represented by theabove-mentioned general formula (I) includebis(3-triethoxysilylpropyl)tetrasulfide,bis(3-trimethoxysilylpropyl)tetrasulfide,bis(3-methyldimethoxysilylpropyl)tetrasulfide,bis(2-triethoxysilylethyl)tetrasulfide,bis(3-triethoxysilylpropyl)disulfide,bis(3-trimethoxysilylpropyl)disulfide,bis(3-methyldimethoxysilylpropyl)disulfide,bis(2-triethoxysilylethyl)disulfide,bis(3-triethoxysilylpropyl)trisulfide,bis(3-trimethoxysilylpropyl)trisulfide,bis(3-methyldimethoxysilylpropyl)trisulfide,bis(2-triethoxysilylethyl)trisulfide,bis(3-monoethoxydimethylsilylpropyl)tetrasulfide,bis(3-monoethoxydimethylsilylpropyl)trisulfide,bis(3-monoethoxydimethylsilylpropyl)disulfide,bis(3-monomethoxydimethylsilylpropyl)tetrasulfide,bis(3-monomethoxydimethylsilylpropyl)trisulfide,bis(3-monomethoxydimethylsilylpropyl)disulfide,bis(2-monoethoxydimethylsilylethyl)tetrasulfide,bis(2-monoethoxydimethylsilylethyl)trisulfide,bis(2-monoethoxydimethylsilylethyl)disulfide.

wherein R⁴ represents a monovalent group selected from —Cl, —Br, R⁹O—,R⁹C(═O)O—, R⁹R¹⁰C═NO—, R⁹R¹⁰CNO—, R⁹R¹⁰N—, and—(OSiR⁹R¹⁰)_(h)(OSiR⁹R¹⁰R¹¹) (where R⁹, R¹⁰ and R¹¹, which may be thesame or different, each represent a hydrogen atom or a monovalenthydrocarbon group having from 1 to 18 carbon atoms; and h indicates from1 to 4 as a mean value); R⁵ represents R⁴, a hydrogen atom, or amonovalent hydrocarbon group having from 1 to 18 carbon atoms; R⁶represents R⁴, R⁵, a hydrogen atom, or a group —[O(R¹²O)_(j)]_(0.5)(where R¹² represents an alkylene group having from 1 to 18 carbonatoms; and j indicates an integer of from 1 to 4); R⁷ represents adivalent hydrocarbon group having from 1 to 18 carbon atoms; R⁸represents a monovalent hydrocarbon group having from 1 to 18 carbonatoms; x, y and z each indicate a number satisfying the relationship ofx+y+2z=3, 0≦x≦3, 0≦y≦2, 0≦z≦1.

In the general formula (II), R⁸, R⁹, R¹⁰ and R¹¹, which may be the sameor different, each preferably represent a group selected from the groupconsisting of a linear, cyclic or branched, alkyl, alkenyl, aryl andaralkyl groups having from 1 to 18 carbon atoms. In case where R⁵ is amonovalent hydrocarbon group having from 1 to 18 carbon atoms, the groupis preferably a group selected from the group consisting of a linear,cyclic or branched, alkyl, alkenyl, aryl and aralkyl groups. Preferably,R¹² is a linear, cyclic or branched alkylene group, and is morepreferably a linear one. R⁷ is, for example, an alkylene group havingfrom 1 to 18 carbon atoms, an alkenylene group having from 2 to 18carbon atoms, a cycloalkylene group having from 5 to 18 carbon atoms, acycloalkylalkylene group having from 6 to 18 carbon atoms, an arylenegroup having from 6 to 18 carbon atoms, or an aralkylene group havingfrom 7 to 18 carbon atoms. The alkylene group and the alkenylene groupmay be linear or branched; and the cycloalkylene group, thecycloalkylalkylene group, the arylene group and the aralkylene group mayhave a substituent such as a lower alkyl group or the like on the ringthereof. Preferably, R⁷ is an alkylene group having from 1 to 6 carbonatoms, particularly preferably a linear alkylene group, for example, amethylene group, an ethylene group, a trimethylene group, atetramethylene group, a pentamethylene group or a hexamethylene group.

Specific examples of the monovalent hydrocarbon group having from 1 to18 carbon atoms of R⁵, R⁸, R⁹, R¹⁰ and R¹¹ in the general formula (II)include a methyl group, an ethyl group, an n-propyl group, an isopropylgroup, an n-butyl group, an isobutyl group, a sec-butyl group, atert-butyl group, a pentyl group, a hexyl group, an octyl group, a decylgroup, a dodecyl group, a cyclopentyl group, a cyclohexyl group, a vinylgroup, a propenyl group, an allyl group, a hexenyl group, an octenylgroup, a cyclopentenyl group, a cyclohexenyl group, a phenyl group, atolyl group, a xylyl group, a naphthyl group, a benzyl group, aphenethyl group, a naphthylmethyl group, etc.

Examples of R¹² in the general formula (II) include a methylene group,an ethylene group, a trimethylene group, a tetramethylene group, apentamethylene group, a hexamethylene group, an octamethylene group, adecamethylene group, a dodecamethylene group, etc.

Specific examples of the silane coupling agent (C) represented by thegeneral formula (II) include 3-hexanoylthiopropyltriethoxysilane,3-octanoylthiopropyltriethoxysilane,3-decanoylthiopropyltriethoxysilane, 3-lauroylthiopropyltriethoxysilane,2-hexanoylthioethyltriethoxysilane, 2-octanoylthioethyltriethoxysilane,2-decanoylthioethyltriethoxysilane, 2-lauroylthioethyltriethoxysilane,3-hexanoylthiopropyltrimethoxysilane,3-octanoylthiopropyltrimethoxysilane,3-decanoylthiopropyltrimethoxysilane,3-lauroylthiopropyltrimethoxysilane,2-hexanoylthioethyltrimethoxysilane,2-octanoylthioethyltrimethoxysilane,2-decanoylthioethyltrimethoxysilane, 2-lauroylthioethyltrimethoxysilane,etc. Of those, especially preferred is3-octanoylthiopropyltriethoxysilane (“NXT Silane” (registered tradename), produced by Momentive Performance Materials Inc.).[Chem. 3](R¹³O)_(3-s)(R¹⁴)_(s)Si—R¹⁵—S_(k)—R¹⁶—S_(k)—R¹⁵—Si(OR¹³)_(3-t)(R¹⁴)_(t)  (III)wherein R¹³, which may be the same or different, each represents alinear, cyclic or branched alkyl group, having from 1 to 8 carbon atomsor a linear or branched alkoxylalkyl group, having from 2 to 8 carbonatoms; R¹⁴, which may be the same or different, each represents alinear, cyclic or branched alkyl group, having from 1 to 8 carbon atoms;R¹⁵, which may be the same or different, each represents a linear orbranched alkylene group, having from 1 to 8 carbon atoms; R¹⁶ representsa divalent group of a general formula (—S—R¹⁷—S—), (—R¹⁸—S_(m1)—R¹⁹—) or(—R²⁰—S_(m2)—R²¹—S_(m3)—R²²—) (where R¹⁷ to R²², which may be the sameor different, each represent a divalent hydrocarbon group, a divalentaromatic group or a divalent organic group containing a hetero elementexcept sulfur and oxygen, having from 1 to 20 carbon atoms; m1, m2 andm3 may be the same or different, each indicating from 1 to less than 4as a mean value); k, which may be the same or different, each indicatefrom 1 to 6 as a mean value; s and t, which may be the same ordifferent, each indicate from 0 to 3 as a mean value, provided that boths and t are not 3 at the same time.

Preferred examples of the silane coupling agent (C) represented by theabove-mentioned general formula (III) are compounds represented by anaverage compositional formula(CH₃CH₂O)₃Si—(CH₂)₃—S₂—(CH₂)₆—S₂—(CH₂)₃—Si(OCH₂CH₃)₃, an averagecompositional formula(CH₃CH₂O)₃Si—(CH₂)₃—S₂—(CH₂)₁₀—S₂—(CH₂)₃—Si(OCH₂CH₃)₃, an averagecompositional formula(CH₃CH₂O)₃Si—(CH₂)₃—S₃—(CH₂)₆—S₃—(CH₂)₃—Si(OCH₂CH₃)₃, an averagecompositional formula(CH₃CH₂O)₃Si—(CH₂)₃—S₄—(CH₂)₆—S₄—(CH₂)₃—Si(OCH₂CH₃)₃, an averagecompositional formula(CH₃CH₂O)₃Si—(CH₂)₃—S—(CH₂)₆—S₂—(CH₂)₆—S—(CH₂)₃—Si(OCH₂CH₃)₃, an averagecompositional formula(CH₃CH₂O)₃Si—(CH₂)₃—S—(CH₂)₆—S_(2.5)—(CH₂)₆—S—(CH₂)₃—Si(OCH₂CH₃)₃, anaverage compositional formula(CH₃CH₂O)₃Si—(CH₂)₃—S—(CH₂)₆—S₃—(CH₂)₆—S—(CH₂)₃—Si(OCH₂CH₃)₃, an averagecompositional formula(CH₃CH₂O)₃Si—(CH₂)₃—S—(CH₂)₆—S₄—(CH₂)₆—S—(CH₂)₃—Si(OCH₂CH₃)₃, an averagecompositional formula(CH₃CH₂O)₃Si—(CH₂)₃—S—(CH₂)₁₀—S₂—(CH₂)₁₀—S—(CH₂)₃—Si(OCH₂CH₃)₃, anaverage compositional formula(CH₃CH₂O)₃Si—(CH₂)₃—S₄—(CH₂)₆—S₄—(CH₂)₆—S₄—(CH₂)₃—Si(OCH₂CH₃)₃, anaverage compositional formula(CH₃CH₂O)₃Si—(CH₂)₃—S₂—(CH₂)₆—S₂—(CH₂)₆—S₂—(CH₂)₃—Si(OCH₂CH₃)₃, anaverage compositional formula(CH₃CH₂O)₃Si—(CH₂)₃—S—(CH₂)₆—S₂—(CH₂)₆—S₂—(CH₂)₆—S—(CH₂)₃—Si(OC H₂CH₃)₃,etc. The synthetic example of the silane coupling agent (C) representedby the above general formula (III) is described, for example, inWO2004/000930.

wherein R²³ represents a linear, branched or cyclic alkyl group, havingfrom 1 to 20 carbon atoms; G, which may be the same or different, eachrepresent an alkanediyl group or an alkenediyl group, having from 1 to 9carbon atoms; Z^(a), which may be the same or different, each representa group capable of bonding to the two silicon atoms and selected from[—O—]_(0.5), [—O-G-]_(0.5) and [—O-G-O—]_(0.5); Z^(b), which may be thesame or different, each represent a group which is capable of bonding tothe two silicon atoms and is the group represented by [—O-G-O-]_(0.5);Z^(c), which may be the same or different, each represent a functionalgroup selected from —Cl, —Br, —OR^(a), R^(a)C(═O)O—, R^(a)R^(b)C═NO—,R^(a)R^(b)N—, R^(a)— and HO-G-O— (where G is the same as above); R^(a)and R^(b), which may be the same or different, each represent a linear,branched or cyclic alkyl group having from 1 to 20 carbon atoms; m, n,u, v and w, which may be the same or different, each are 1≦m≦20, 0≦n≦20,0≦u≦3, 0≦v≦2, 0≦w≦1, and (u/2)+v+2w=2 or 3; in case where the formulahas multiple A's, then Z^(a) _(u), Z^(b) _(v) and Z^(c) _(w) may be thesame or different in those multiple A's; in case where the formula hasmultiple B's, then Z^(a) _(u), Z^(b) _(v) and Z^(c) _(w) may be the sameor different in those multiple B's.

Specific examples of the silane coupling agent (C) represented by thegeneral formula (IV) include the following chemical formula (V),chemical formula (VI) and chemical formula (VII):

In the formula, L each independently represents an alkanediyl group oran alkenediyl group having from 1 to 9 carbon atoms; and x=m and y=n.

As the silane coupling agent represented by the chemical formula (V), acommercial product is available as “NXT Low-V Silane” (registered tradename), produced by Momentive Performance Materials Inc.

As the silane coupling agent represented by the chemical formula (VI), acommercial product is available as “NXT Ultra Low-V Silane” (registeredtrade name), produced by Momentive Performance Materials Inc.

Further, as the silane coupling agent represented by the chemicalformula (VII), there is mentioned a commercial product of “NXT-Z”(registered trade name), produced by Momentive Performance MaterialsInc.

The silane coupling agent represented by the general formula (II), thechemical formula (V) or the chemical formula (VI) has a protectedmercapto group, and is therefore effective for preventing initialscorching in the processing process before the vulcanization step, andaccordingly, the processability thereof is good.

In the silane coupling agent represented by the general formula (V),(VI) or (VII), the carbon number of the alkoxysilane is large, andtherefore the amount of the volatile compound VOC (especially alcohol)to be generated from the agent is small, and accordingly, the agent isfavorable in point of working environment. Further, the silane couplingagent of the chemical formula (VII) provides a low-heat-generationproperty as tire performance, and is therefore more preferred.

The silane coupling agent (C) in the present invention is particularlypreferably the compound represented by the general formula (I) among thecompounds represented by the general formulae (I) to (IV). This isbecause in the case where the vulcanization accelerator (D) is added inthe first step of kneading for enhancing the activity of the silanecoupling agent (C), the activation of the polysulfide-bonding site to bereacted with the rubber component (A) is facilitated.

In the present invention, the silane coupling agent (C) may be usedsolely or as a combination of two or more kinds thereof.

The mixing amount of the silane coupling agent (C) in the rubbercomposition of the present invention is preferably from 1 to 20% by massbased on the silica. This is because when the amount is less than 1% bymass, it may be difficult to enhance the low-heat-generation property ofthe rubber composition, and when the amount exceeds 20% by mass, thecost of the rubber composition may be too increased to lower theeconomical efficiency. The amount is more preferably from 3 to 20% bymass based on the silica, and particularly preferably from 4 to 10% bymass based on the silica.

In the case where the silica (B) having a CTAB adsorption specificsurface area of 180 m²/g or more is used in the rubber composition ofthe present invention, for providing an average aggregated aggregatearea (nm²) of the silica in the rubber composition after vulcanizationof 1,700 or less, the production method of the rubber composition is notlimited, and the rubber composition may be produced by any kneadingmethod, but the following production methods (1) to (5) are preferredsince the rubber composition may be produced with an ordinary equipmentwith high productivity.

(1) A production method of the rubber composition by kneading the rubbercomposition through plural steps, in which the rubber component (A), thewhole or a part of the silica (B), the whole or a part of the silanecoupling agent (C) and the vulcanization accelerator (D) are kneaded inthe first step of kneading, where the molar amount of the organic acidcompound in the rubber composition in the first step is limited to 1.5times or less the molar amount of the vulcanization accelerator (D). Inthis case, the vulcanization accelerator (D) is preferably at least oneselected from a guanidine compound, a sulfenamide compound and athiazole compound.

(2) A production method of the rubber composition by kneading the rubbercomposition through plural steps, in which the rubber component (A), thewhole or a part of the silica (B) and the whole or a part of the silanecoupling agent (C) are kneaded in the first step of kneading, and thevulcanization accelerator (D) is added in the course of the first step,followed by further kneading. In this case, the vulcanizationaccelerator (D) is preferably at least one selected from a guanidinecompound, a sulfenamide compound, a thiazole compound, a thiuramcompound, a dithiocarbamate salt compound, a thiourea compound and axanthogenate salt compound.

(3) A production method of the rubber composition by kneading the rubbercomposition through three or more kneading steps, in which the rubbercomponent (A), the whole or a part of the silica (B) and the whole or apart of the silane coupling agent (C) are kneaded in the first step (X)of kneading; the vulcanization accelerator (D) is added and kneaded inthe step (Y) that is the second or later step and before the final stepof kneading; and the vulcanization agent is added and kneaded in thefinal step (Z) of kneading. In this case, the vulcanization accelerator(D) is preferably at least one selected from a guanidine compound, asulfenamide compound, a thiazole compound, a thiuram compound, adithiocarbamate salt compound, a thiourea compound and a xanthogenatesalt compound.

(4) A production method of the rubber composition by kneading the rubbercomposition through plural steps, in which the rubber component (A), thewhole or a part of the silica (B), the whole or a part of the silanecoupling agent (C) and the vulcanization accelerator (D) are kneaded inthe first step of kneading. In this case, the vulcanization accelerator(D) is preferably at least one selected from a guanidine compound, asulfenamide compound, a thiazole compound, a thiuram compound, adithiocarbamate salt compound, a thiourea compound and a xanthogenatesalt compound. In the method (4), the following production method (5) ispreferably performed.

(5) A production method of the rubber composition by kneading the rubbercomposition through plural steps, in which the rubber component (A), thewhole or a part of the silica (B), the whole or a part of the silanecoupling agent (C) and the vulcanization accelerator (D) are kneaded inthe first step of kneading, where the molar amount of the organic acidcompound in the rubber composition in the first step is limited to 1.5times or less the molar amount of the vulcanization accelerator (D). Inthis case, the vulcanization accelerator (D) is preferably at least oneselected from a guanidine compound, a sulfenamide compound, a thiazolecompound, a thiuram compound, a dithiocarbamate salt compound, athiourea compound and a xanthogenate salt compound.

In the production methods (1) to (5), the step of kneading before thefinal step, such as the first step and the second step, is a processstep where the raw materials, such as the rubber component, the fillerand the coupling agent, other than the reagents that contribute tocrosslinking (e.g., the vulcanizing agent and vulcanization accelerator)are mixed and kneaded, and is a process step for performing dispersionof the filler to the rubber composition for reinforcing the rubbercomponent. The step of kneading that is the second or later step andbefore the final step does not include a step of kneading that performsonly kneading without addition of any raw material and does not includea special mixing method, such as a wet master batch.

The maximum temperature of the rubber composition in the step ofkneading before the final step, such as the first step and the secondstep, is preferably from 120 to 190° C., more preferably from 130 to175° C., and further preferably from 150 to 170° C. The kneading time ispreferably from 0.5 to 20 minutes, more preferably from 0.5 to 10minutes, and further preferably from 0.5 to 5 minutes.

The final step of kneading is a process step where the reagents thatcontribute to crosslinking (e.g., the vulcanizing agent andvulcanization accelerator) are mixed and kneaded. The maximumtemperature of the rubber composition in the final step is preferablyfrom 60 to 140° C., more preferably from 80 to 120° C., and furtherpreferably from 100 to 120° C. The kneading time is preferably from 0.5to 20 minutes, more preferably from 0.5 to 10 minutes, and furtherpreferably from 0.5 to 5 minutes.

[Vulcanization Accelerator (D)]

As the vulcanization accelerator (D) which can be used for the rubbercomposition of the present invention, preferred examples includeguanidines, sulfenamides, thiazoles, thiurams, dithiocarbamate salts,thioureas and xanthate salts.

The guanidines for use in the rubber composition of the presentinvention include 1,3-diphenylguanidine, 1,3-di-o-tolylguanidine,1-o-tolylbiguanide, dicatechol borate di-o-tolylguanidine salt,1,3-di-o-cumenylguanidine, 1,3-di-o-biphenylguanidine,1,3-di-o-cumenyl-2-propionylguanidine, etc. Preferred are1,3-diphenylguanidine, 1,3-di-o-tolylguanidine and 1-o-tolylbiguanide ashaving high reactivity.

The sulfenamides for use in the rubber composition of the presentinvention include N-cyclohexyl-2-benzothiazolylsulfenamide,N,N-dicyclohexyl-2-benzothiazolylsulfenamide,N-tert-butyl-2-benzothiazolylsulfenamide,N-oxydiethylene-2-benzothiazolylsulfenamide,N-methyl-2-benzothiazolylsulfenamide,N-ethyl-2-benzothiazolylsulfenamide,N-propyl-2-benzothiazolylsulfenamide,N-butyl-2-benzothiazolylsulfenamide,N-pentyl-2-benzothiazolylsulfenamide,N-hexyl-2-benzothiazolylsulfenamide,N-pentyl-2-benzothiazolylsulfenamide,N-octyl-2-benzothiazolylsulfenamide,N-2-ethylhexyl-2-benzothiazolylsulfenamide,N-decyl-2-benzothiazolylsulfenamide,N-dodecyl-2-benzothiazolylsulfenamide,N-stearyl-2-benzothiazolylsulenamide,N,N-dimethyl-2-benzothiazolylsulenamide,N,N-diethyl-2-benzothiazolylsulenamide,N,N-dipropyl-2-benzothiazolylsulenamide,N,N-dibutyl-2-benzothiazolylsulenamide,N,N-dipentyl-2-benzothiazolylsulenamide,N,N-dihexyl-2-benzothiazolylsulenamide,N,N-dipentyl-2-benzothiazolylsulenamide,N,N-dioctyl-2-benzothiazolylsulenamide,N,N-di-2-ethylhexylbenzothiazolylsulfenamide,N-decyl-2-benzothiazolylsulenamide,N,N-didodecyl-2-benzothiazolylsulenamide,N,N-distearyl-2-benzothiazolylsulenamide, etc. Of those, preferred areN-cyclohexyl-2-benzothiazolylsulenamide andN-tert-butyl-2-benzothiazolylsulenamide, as having high reactivity.

The thiazoles for use in the rubber composition of the present inventioninclude 2-mercaptobenzothiazole, di-2-benzothiazolyl disulfide,2-mercaptobenzothiazole zinc salt, 2-mercaptobenzothiazolecyclohexylamine salt, 2-(N,N-diethylthiocarbamoylthio)benzothiazole,2-(4′-morpholinodithio)benzothiazole, 4-methyl-2-mercaptobenzothiazole,di-(4-methyl-2-benzothiazolyl)disulfide,5-chloro-2-mercaptobenzothiazole, 2-mercaptobenzothiazole sodium,2-mercapto-6-nitrobenzothiazole, 2-mercapto-naphtho[1,2-d]thiazole,2-mercapto-5-methoxybenzothiazole, 6-amino-2-mercaptobenzothiazole, etc.Of those, preferred are 2-mercaptobenzothiazole and di-2-benzothiazolyldisulfide, as having high reactivity.

The thiurams for use in the rubber composition of the present inventioninclude tetramethylthiuram disulfide, tetraethylthiuram disulfide,tetrapropylthiuram disulfide, tetraisopropylthiuram disulfide,tetrabutylthiuram disulfide, tetrapentylthiuram disulfide,tetrahexylthiuram disulfide, tetraheptylthiuram disulfide,tetraoctylthiuram disulfide, tetranonylthiuram disulfide,tetradecylthiuram disulfide, tetradodecylthiuram disulfide,tetrastearylthiuram disulfide, tetrabenzylthiuram disulfide,tetrakis(2-ethylhexyl)thiuram disulfide, tetramethylthiuram monosulfide,tetraethylthiuram monosulfide, tetrapropylthiuram monosulfide,tetraisopropylthiuram monosulfide, tetrabutylthiuram monosulfide,tetrapentylthiuram monosulfide, tetrahexylthiuram monosulfide,tetraheptylthiuram monosulfide, tetraoctylthiuram monosulfide,tetranonylthiuram monosulfide, tetradecylthiuram monosulfide,tetradodecylthiuram monosulfide, tetrastearylthiuram monosulfide,tetrabenzylthiuram monosulfide, dipentamethylenethiuram tetrasulfide,etc. Of those, preferred are tetrakis(2-ethylhexyl)thiuram disulfide andtetrabenzylthiuram disulfide, as having high reactivity.

The dithiocarbamate salts for use in the rubber composition of thepresent invention include zinc dimethyldithiocarbamate, zincdiethyldithiocarbamate, zinc dipropyldithiocarbamate, zincdiisopropyldithiocarbamate, zinc dibutyldithiocarbamate, zincdipentyldithiocarbamate, zinc dihexyldithiocarbamate, zincdiheptyldithiocarbamate, zinc dioctyldithiocarbamate, zincdi(2-ethylhexyl)dithiocarbamate, zinc didecyldithiocarbamate, zincdidodecyldithiocarbamate, zinc N-pentamethylenedithiocarbamate, zincN-ethyl-N-phenyldithiocarbamate, zinc dibenzyldithiocarbamate, copperdimethyldithiocarbamate, copper diethyldithiocarbamate, copperdipropyldithiocarbamate, copper diisopropyldithiocarbamate, copperdibutyldithiocarbamate, copper dipentyldithiocarbamate, copperdihexyldithiocarbamate, copper diheptyldithiocarbamate, copperdioctyldithiocarbamate, copper di(2-ethylhexyl)dithiocarbamate, copperdidecyldithiocarbamate, copper didodecyldithiocarbamate, copper N-pentamethylenedithiocarbamate, copper dibenzyldithiocarbamate, sodiumdimethyldithiocarbamate, sodium diethyldithiocarbamate, sodiumdipropyldithiocarbamate, sodium diisopropyldithiocarbamate, sodiumdibutyldithiocarbamate, sodium dipentyldithiocarbamate, sodiumdihexyldithiocarbamate, sodium diheptyldithiocarbamate, sodiumdioctyldithiocarbamate, sodium di(2-ethylhexyl)dithiocarbamate, sodiumdidecyldithiocarbamate, sodium didodecyldithiocarbamate, sodium N-pentamethylenedithiocarbamate, sodium dibenzyldithiocarbamate, ferricdimethyldithiocarbamate, ferric diethyldithiocarbamate, ferricdipropyldithiocarbamate, ferric diisopropyldithiocarbamate, ferricdibutyldithiocarbamate, ferric dipentyldithiocarbamate, ferricdihexyldithiocarbamate, ferric diheptyldithiocarbamate, ferricdioctyldithiocarbamate, ferric di(2-ethylhexyl)dithiocarbamate, ferricdidecyldithiocarbamate, ferric didodecyldithiocarbamate, ferric N-pentamethylenedithiocarbamate, ferric dibenzyldithiocarbamate, etc. Ofthose, preferred are zinc dibenzyldithiocarbamate, zincN-ethyl-N-phenyldithiocarbamate, zinc dimethyldithiocarbamate and copperdimethyldithiocarbamate, as having high reactivity.

The thioureas for use in the rubber composition of the present inventioninclude N,N′-diphenylthiourea, trimethylthiourea, N,N′-diethylthiourea,N,N′-dimethylthiourea, N,N′-dibutylthiourea, ethylenethiourea,N,N′-diisopropylthiourea, N,N′-dicyclohexylthiourea,1,3-di(o-tolyl)thiourea, 1,3-di(p-tolyl)thiourea,1,1-diphenyl-2-thiourea, 2,5-dithiobiurea, guanylthiourea,1-(1-naphthyl)-2-thiourea, 1-phenyl-2-thiourea, p-tolylthiourea,o-tolylthiourea, etc. Of those, preferred are N,N′-diethylthiourea,trimethylthiourea, N,N′-diphenylthiourea and N,N′-dimethylthiourea, ashaving high reactivity.

The xanthate salts for use in the rubber composition of the presentinvention include zinc methylxanthate, zinc ethylxanthate, zincpropylxanthate, zinc isopropylxanthate, zinc butylxanthate, zincpentylxanthate, zinc hexylxanthate, zinc heptylxanthate, zincoctylxanthate, zinc 2-ethylhexylxanthate, zinc decylxanthate, zincdodecylxanthate, potassium methylxanthate, potassium ethylxanthate,potassium propylxanthate, potassium isopropylxanthate, potassiumbutylxanthate, potassium pentylxanthate, potassium hexylxanthate,potassium heptylxanthate, potassium octylxanthate, potassium2-ethylhexylxanthate, potassium decylxanthate, potassiumdodecylxanthate, sodium methylxanthate, sodium ethylxanthate, sodiumpropylxanthate, sodium isopropylxanthate, sodium butylxanthate, sodiumpentylxanthate, sodium hexylxanthate, sodium heptylxanthate, sodiumoctylxanthate, sodium 2-ethylhexylxanthate, sodium decylxanthate, sodiumdodecylxanthate, etc. Of those, preferred is zinc isopropylxanthate, ashaving high reactivity.

The rubber composition of the present invention preferably contains thevulcanization accelerator (D) in an amount of from 0.1 to 10 parts bymass, and more preferably from 0.2 to 7 parts by mass, per 100 parts bymass of the rubber component (A). Of those, from 0.1 to 5 parts by massof the vulcanization accelerator (D) is preferably added in the stepbefore the final step of kneading, and from 0.1 to 5 parts by massthereof is preferably added in the final step of kneading.

[Organic Acid Compound]

Examples of the organic acid compound added to the rubber composition ofthe present invention include an organic acid selected from a saturatedfatty acid and an unsaturated fatty acid, such as stearic acid, palmiticacid, myristic acid, lauric acid, arachidinic acid, behenic acid,lignoceric acid, capric acid, pelargonic acid, caprylic acid, enanthicacid, caproic acid, oleic acid, vaccenic acid, linoleic acid, linolenicacid and nervonic acid, and a resin acid, such as resin acid andmodified resin acid, a metal salt and an ester of the organic acid, anda phenol derivative.

In the present invention, it is necessary to exhibit the function of thevulcanization accelerator sufficiently, and therefore, 50% by mol ormore of the organic acid compound is preferably stearic acid.

In the rubber composition of the present invention, various additivesthat are generally incorporated in a rubber composition, for example, avulcanization activator such as zinc flower or the like, an anti-agingagent and others may be optionally added and kneaded in the first stageor the final stage of kneading, or in the intermediate stage between thefirst stage and the final stage.

As the kneading apparatus for the present invention, usable is any of aBanbury mixer, a roll, an intensive mixer, etc.

EXAMPLE

The present invention will be described in more detail with reference toexamples below, but the present invention is not limited to theexamples.

The average aggregated aggregate area and the low-heat-generationproperty (tan δ index) of the vulcanized rubber composition wereevaluated in the following manners.

<Average Aggregated Aggregate Area of Vulcanized Rubber Composition>

A specimen of the vulcanized rubber composition was produced by cuttinga vulcanized rubber sheet with a razor. The shape was 5 mm×5 mm×1 mm(thickness).

The upper surface of the specimen was cut in a direction making an angleof 38° with respect to the upper surface of the specimen with a focusedion beam under condition of a voltage of 30 kV by using FIB-SEM (NOVA200, produced by FEI Company). The smooth surface of the specimen formedby cutting was imaged with an SEM at an acceleration voltage of 5 kV ina direction perpendicular to the smooth surface. The resulting image wasconverted to a binarized image of the rubber portion and the silicaportion as a filler of the specimen by the Otsu's method, an aggregatedaggregate area of the silica portion was obtained based on the resultingbinarized image, and the average aggregated aggregate area wascalculated in terms of number average (arithmetic average) per unit area(3 μm×3 μm) from the total surface area of the silica portion and thenumber of aggregated aggregates. In the calculation, a particle that wasin contact with an edge of the image was not counted, and a particle of20 pixels or less was assumed to be noise and was not counted.

<Low-Heat-Generation Property (Tan δ Index)>

Using a viscoelasticity measuring device (produced by RheometricScientific, Inc.), tan δ was measured at a temperature of 60° C., adynamic strain of 5% and a frequency of 15 Hz. Based on the reciprocalof tan δ in Comparative Example 1, 6, 10, 15, 20, 25, 30, 34, 38 or 42as referred to 100, the data were expressed as index indicationaccording to the following expression. A larger index value means betterlow-heat-generation property and a smaller hysteresis loss.Low-Heat-Generation Index={(tan δ of vulcanized rubber composition ofComparative Example 1, 6, 10, 15, 20, 25, 30, 34, 38 or 42)/(tan δ ofvulcanized rubber composition tested)}×100

The raw materials used in Examples 1 to 40 and Comparative Examples 1 to45 are abbreviated as follows.

(1) Emulsion-polymerized SBR-1: emulsion-polymerized styrene-butadienecopolymer rubber (SBR), “#1500”, a trade name, produced by JSRCorporation

(2) Solution-polymerized SBR-2: unmodified solution-polymerizedstyrene-butadiene copolymer rubber (SBR), “Tufdene 2000”, a trade name,produced by Asahi Kasei Corporation

(3) Natural rubber: RSS #3

(4) Carbon black N220: “#80”, a trade name, produced by Asahi CarbonCo., Ltd.

(5) Silica-1: “Zeosil HRS 1200”, a trade name (registered trademark),produced by Rhodia, Inc. (CTAB adsorption specific surface area: 200m²/g)

(6) Silane coupling agent Si75: bis(3-triethoxysilylpropyl)disulfide(average sulfur chain length: 2.35), silane coupling agent, “Si75”, atrade name (registered trademark), produced by Evonik Industries AG

(7) Anti-aging agent 6PPD:N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine, “Nocrac 6C”, a tradename, produced by Ouchi Shinko Chemical Industrial Co., Ltd.

(8) 1,3-Diphenylguanidine: “Sanceler D”, a trade name, produced bySanshin Chemical Industry Co., Ltd.

(9) Anti-aging agent TMDQ: 2,2,4-trimethyl-1,2-dihydroquinoline polymer,“Nocrac 224”, a trade name, produced by Ouchi Shinko Chemical IndustrialCo., Ltd.

(10) Vulcanization accelerator MBTS: di-2-benzothiazolyl disulfide,“Sanceler DM”, a trade name, produced by Sanshin Chemical Industry Co.,Ltd.

(11) Vulcanization accelerator TBBS:N-tert-butyl-2-benzothiazolylsulfenamide, “Sanceler NS”, a trade name,produced by Sanshin Chemical Industry Co., Ltd.

Example 1

In the first step of kneading, with a Banbury mixer, 25 parts by mass ofthe emulsion-polymerized SBR-1 and 75 parts by mass of thesolution-polymerized SBR-2 as the rubber component (A), 10 parts by massof the carbon black N220, 50 parts by mass of the silica-1 as the silica(B), 4 parts by mass of the silane coupling agent Si75 as the silanecoupling agent (C) and 30 parts by mass of an aromatic oil were kneadedfor 60 seconds, and then 1 part by mass of 1,3-diphenylguanidine, whichis a guanidine compound, as the vulcanization accelerator (D) was addedand further kneaded, in which the maximum temperature of the rubbercomposition in the first step of kneading was regulated to 150° C.

Subsequently, in the final step of kneading, 2 parts by mass of stearicacid, 1 part by mass of the anti-aging agent 6PPD, 1 part by mass of theanti-aging agent TMDQ, 2.5 parts by mass of zinc flower, 0.6 parts bymass of 1,3-diphenylguanidine, 1 part by mass of the vulcanizationaccelerator MBTS, 0.6 parts by mass of the vulcanization acceleratorTBBS and 1.5 parts by mass of sulfur were added, in which the maximumtemperature of the rubber composition in the final step of kneading wasregulated to 110° C.

The vulcanized rubber composition obtained from the rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 1.

Example 2

The kneading operation was performed in the same manner as in Example 1,except that 50 parts by mass of the emulsion-polymerized SBR-1 and 50parts by mass of the solution-polymerized SBR-2 were used as the rubbercomponent (A). The resulting vulcanized rubber composition was evaluatedfor the average aggregated aggregate area and the low-heat-generationproperty (tan δ index) according to the aforementioned manners. Theresults are shown in Table 1.

Example 3

The kneading operation was performed in the same manner as in Example 1,except that 67 parts by mass of the emulsion-polymerized SBR-1 and 33parts by mass of the solution-polymerized SBR-2 were used as the rubbercomponent (A). The resulting vulcanized rubber composition was evaluatedfor the average aggregated aggregate area and the low-heat-generationproperty (tan δ index) according to the aforementioned manners. Theresults are shown in Table 1.

Example 4

The kneading operation was performed in the same manner as in Example 1,except that 100 parts by mass of the emulsion-polymerized SBR-1 was usedas the rubber component (A), and the maximum temperature of the rubbercomposition in the first step of kneading was regulated to 170° C. Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 1.

Comparative Example 1

The kneading operation was performed in the same manner as in Example 1,except that 1 part by mass of 1,3-diphenylguanidine was not added in thefirst step of kneading, neither 2 parts by mass of stearic acid nor 1part by mass of the anti-aging agent 6PPD was added in the final step ofkneading, and 100 parts by mass of the solution-polymerized SBR-2 wasused as the rubber component (A), and 2 parts by mass of stearic acidand 1 part by mass of the anti-aging agent 6PPD were added, in the firststep of kneading. The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 1.

Comparative Example 2

The kneading operation was performed in the same manner as inComparative Example 1, except that 40 parts by mass of theemulsion-polymerized SBR-1 and 60 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 1.

Comparative Example 3

The kneading operation was performed in the same manner as inComparative Example 1, except that 50 parts by mass of theemulsion-polymerized SBR-1 and 50 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 1.

Comparative Example 4

The kneading operation was performed in the same manner as inComparative Example 1, except that 67 parts by mass of theemulsion-polymerized SBR-1 and 33 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 1.

Comparative Example 5

The kneading operation was performed in the same manner as inComparative Example 1, except that 100 parts by mass of theemulsion-polymerized SBR-1 was used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 1.

Example 5

The kneading operation was performed in the same manner as in Example 1,except that 25 parts by mass of the natural rubber and 75 parts by massof the solution-polymerized SBR-2 were used as the rubber component (A).The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 2.

Example 6

The kneading operation was performed in the same manner as in Example 1,except that 50 parts by mass of the natural rubber and 50 parts by massof the solution-polymerized SBR-2 were used as the rubber component (A).The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 2.

Example 7

The kneading operation was performed in the same manner as in Example 1,except that 67 parts by mass of the natural rubber and 33 parts by massof the solution-polymerized SBR-2 were used as the rubber component (A).The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 2.

Example 8

The kneading operation was performed in the same manner as in Example 1,except that 100 parts by mass of the natural rubber was used as therubber component (A). The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 2.

Comparative Example 6

The kneading operation was performed in the same manner as inComparative Example 1, except that 25 parts by mass of the naturalrubber and 75 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 2.

Comparative Example 7

The kneading operation was performed in the same manner as inComparative Example 1, except that 50 parts by mass of the naturalrubber and 50 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 2.

Comparative Example 8

The kneading operation was performed in the same manner as inComparative Example 1, except that 67 parts by mass of the naturalrubber and 33 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 2.

Comparative Example 9

The kneading operation was performed in the same manner as inComparative Example 1, except that 100 parts by mass of the naturalrubber was used as the rubber component (A). The resulting vulcanizedrubber composition was evaluated for the average aggregated aggregatearea and the low-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 2.

TABLE 1 Example Comparative Example 1 2 3 4 1 2 3 4 5 Average aggregatedaggregate 1600 1650 1650 1600 2000 2000 2000 2000 2000 area ofvulcanized rubber composition (nm²) Vulcanization property: 108 106 105107 100 100 100 100 100 low-heat-generation property (tanδ index)

TABLE 2 Example Comparative Example 5 6 7 8 6 7 8 9 Average aggregatedaggregate 1600 1650 1650 1700 2000 2000 2000 2000 area of vulcanizedrubber composition (nm²) Vulcanization property: 109 107 106 104 100 100100 100 low-heat-generation property (tanδ index)

Example 9

In the first step of kneading, with a Banbury mixer, 25 parts by mass ofthe emulsion-polymerized SBR-1 and 75 parts by mass of thesolution-polymerized SBR-2 as the rubber component (A), 30 parts by massof the carbon black N220, 30 parts by mass of the silica-1 as the silica(B), 2.4 parts by mass of the silane coupling agent Si75 as the silanecoupling agent (C) and 30 parts by mass of an aromatic oil were kneadedfor 60 seconds, and then 1 part by mass of 1,3-diphenylguanidine, whichis a guanidine compound, as the vulcanization accelerator (D) was addedand further kneaded, in which the maximum temperature of the rubbercomposition in the first step of kneading was regulated to 150° C.

Subsequently, in the final step of kneading, 2 parts by mass of stearicacid, 1 part by mass of the anti-aging agent 6PPD, 1 part by mass of theanti-aging agent TMDQ, 2.5 parts by mass of zinc flower, 1 part by massof the vulcanization accelerator MBTS, 0.6 parts by mass of thevulcanization accelerator TBBS and 1.5 parts by mass of sulfur wereadded, in which the maximum temperature of the rubber composition in thefinal step of kneading was regulated to 110° C.

The vulcanized rubber composition obtained from the rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 3.

Example 10

The kneading operation was performed in the same manner as in Example 9,except that 50 parts by mass of the emulsion-polymerized SBR-1 and 50parts by mass of the solution-polymerized SBR-2 were used as the rubbercomponent (A). The resulting vulcanized rubber composition was evaluatedfor the average aggregated aggregate area and the low-heat-generationproperty (tan δ index) according to the aforementioned manners. Theresults are shown in Table 3.

Example 11

The kneading operation was performed in the same manner as in Example 9,except that 67 parts by mass of the emulsion-polymerized SBR-1 and 33parts by mass of the solution-polymerized SBR-2 were used as the rubbercomponent (A). The resulting vulcanized rubber composition was evaluatedfor the average aggregated aggregate area and the low-heat-generationproperty (tan δ index) according to the aforementioned manners. Theresults are shown in Table 3.

Example 12

The kneading operation was performed in the same manner as in Example 9,except that 100 parts by mass of the emulsion-polymerized SBR-1 was usedas the rubber component (A), and the maximum temperature of the rubbercomposition in the first step of kneading was regulated to 170° C. Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 3.

Comparative Example 10

The kneading operation was performed in the same manner as in Example 9,except that 1 part by mass of 1,3-diphenylguanidine was not added in thefirst step of kneading, neither 2 parts by mass of stearic acid nor 1part by mass of the anti-aging agent 6PPD was added in the final step ofkneading, and 100 parts by mass of the solution-polymerized SBR-2 wasused as the rubber component (A), and 2 parts by mass of stearic acidand 1 part by mass of the anti-aging agent 6PPD were added, in the firststep of kneading. The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 3.

Comparative Example 11

The kneading operation was performed in the same manner as inComparative Example 10, except that 40 parts by mass of theemulsion-polymerized SBR-1 and 60 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 3.

Comparative Example 12

The kneading operation was performed in the same manner as inComparative Example 10, except that 50 parts by mass of theemulsion-polymerized SBR-1 and 50 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 3.

Comparative Example 13

The kneading operation was performed in the same manner as inComparative Example 10, except that 67 parts by mass of theemulsion-polymerized SBR-1 and 33 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 3.

Comparative Example 14

The kneading operation was performed in the same manner as inComparative Example 10, except that 100 parts by mass of theemulsion-polymerized SBR-1 was used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 3.

Example 13

In the first step of kneading, with a Banbury mixer, 25 parts by mass ofthe emulsion-polymerized SBR-1 and 75 parts by mass of thesolution-polymerized SBR-2 as the rubber component (A), 10 parts by massof the carbon black N220, 75 parts by mass of the silica-1 as the silica(B), 6 parts by mass of the silane coupling agent Si75 as the silanecoupling agent (C) and 30 parts by mass of an aromatic oil were kneadedfor 60 seconds, and then 1 part by mass of 1,3-diphenylguanidine, whichis a guanidine compound, as the vulcanization accelerator (D) was addedand further kneaded, in which the maximum temperature of the rubbercomposition in the first step of kneading was regulated to 150° C.

Subsequently, in the final step of kneading, 2 parts by mass of stearicacid, 1 part by mass of the anti-aging agent 6PPD, 1 part by mass of theanti-aging agent TMDQ, 2.5 parts by mass of zinc flower, 0.6 parts bymass of 1,3-diphenylguanidine, 1 part by mass of the vulcanizationaccelerator MBTS, 0.6 parts by mass of the vulcanization acceleratorTBBS and 1.5 parts by mass of sulfur were added, in which the maximumtemperature of the rubber composition in the final step of kneading wasregulated to 110° C.

The vulcanized rubber composition obtained from the rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 4.

Example 14

The kneading operation was performed in the same manner as in Example13, except that 50 parts by mass of the emulsion-polymerized SBR-1 and50 parts by mass of the solution-polymerized SBR-2 were used as therubber component (A). The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 4.

Example 15

The kneading operation was performed in the same manner as in Example13, except that 67 parts by mass of the emulsion-polymerized SBR-1 and33 parts by mass of the solution-polymerized SBR-2 were used as therubber component (A). The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 4.

Example 16

The kneading operation was performed in the same manner as in Example13, except that 100 parts by mass of the emulsion-polymerized SBR-1 wasused as the rubber component (A), and the maximum temperature of therubber composition in the first step of kneading was regulated to 170°C. The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 4.

Comparative Example 15

The kneading operation was performed in the same manner as in Example13, except that 1 part by mass of 1,3-diphenylguanidine was not added inthe first step of kneading, neither 2 parts by mass of stearic acid nor1 part by mass of the anti-aging agent 6PPD was added in the final stepof kneading, and 100 parts by mass of the solution-polymerized SBR-2 wasused as the rubber component (A), and 2 parts by mass of stearic acidand 1 part by mass of the anti-aging agent 6PPD were added, in the firststep of kneading. The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 4.

Comparative Example 16

The kneading operation was performed in the same manner as inComparative Example 15, except that 40 parts by mass of theemulsion-polymerized SBR-1 and 60 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 4.

Comparative Example 17

The kneading operation was performed in the same manner as inComparative Example 15, except that 50 parts by mass of theemulsion-polymerized SBR-1 and 50 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 4.

Comparative Example 18

The kneading operation was performed in the same manner as inComparative Example 15, except that 67 parts by mass of theemulsion-polymerized SBR-1 and 33 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 4.

Comparative Example 19

The kneading operation was performed in the same manner as inComparative Example 15, except that 100 parts by mass of theemulsion-polymerized SBR-1 was used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 4.

Example 17

In the first step of kneading, with a Banbury mixer, 25 parts by mass ofthe emulsion-polymerized SBR-1 and 75 parts by mass of thesolution-polymerized SBR-2 as the rubber component (A), 5 parts by massof the carbon black N220, 100 parts by mass of the silica-1 as thesilica (B), 8 parts by mass of the silane coupling agent Si75 as thesilane coupling agent (C) and 40 parts by mass of an aromatic oil werekneaded for 60 seconds, and then 1 part by mass of1,3-diphenylguanidine, which is a guanidine compound, as thevulcanization accelerator (D) was added and further kneaded, in whichthe maximum temperature of the rubber composition in the first step ofkneading was regulated to 150° C.

Subsequently, in the final step of kneading, 2 parts by mass of stearicacid, 1 part by mass of the anti-aging agent 6PPD, 1 part by mass of theanti-aging agent TMDQ, 2.5 parts by mass of zinc flower, 0.9 part bymass of 1,3-diphenylguanidine, 1 part by mass of the vulcanizationaccelerator MBTS, 0.6 part by mass of the vulcanization accelerator TBBSand 1.5 parts by mass of sulfur were added, in which the maximumtemperature of the rubber composition in the final step of kneading wasregulated to 110° C.

The vulcanized rubber composition obtained from the rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 5.

Example 18

The kneading operation was performed in the same manner as in Example17, except that 50 parts by mass of the emulsion-polymerized SBR-1 and50 parts by mass of the solution-polymerized SBR-2 were used as therubber component (A). The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 5.

Example 19

The kneading operation was performed in the same manner as in Example17, except that 67 parts by mass of the emulsion-polymerized SBR-1 and33 parts by mass of the solution-polymerized SBR-2 were used as therubber component (A). The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 5.

Example 20

The kneading operation was performed in the same manner as in Example17, except that 100 parts by mass of the emulsion-polymerized SBR-1 wasused as the rubber component (A), and the maximum temperature of therubber composition in the first step of kneading was regulated to 170°C. The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 5.

Comparative Example 20

The kneading operation was performed in the same manner as in Example17, except that 1 part by mass of 1,3-diphenylguanidine was not added inthe first step of kneading, neither 2 parts by mass of stearic acid nor1 part by mass of the anti-aging agent 6PPD was added in the final stepof kneading, and 100 parts by mass of the solution-polymerized SBR-2 wasused as the rubber component (A), and 2 parts by mass of stearic acidand 1 part by mass of the anti-aging agent 6PPD were added, in the firststep of kneading. The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 5.

Comparative Example 21

The kneading operation was performed in the same manner as inComparative Example 20, except that 40 parts by mass of theemulsion-polymerized SBR-1 and 60 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 5.

Comparative Example 22

The kneading operation was performed in the same manner as inComparative Example 20, except that 50 parts by mass of theemulsion-polymerized SBR-1 and 50 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 5.

Comparative Example 23

The kneading operation was performed in the same manner as inComparative Example 20, except that 67 parts by mass of theemulsion-polymerized SBR-1 and 33 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 5.

Comparative Example 24

The kneading operation was performed in the same manner as inComparative Example 20, except that 100 parts by mass of theemulsion-polymerized SBR-1 was used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 5.

Example 21

In the first step of kneading, with a Banbury mixer, 25 parts by mass ofthe emulsion-polymerized SBR-1 and 75 parts by mass of thesolution-polymerized SBR-2 as the rubber component (A), 5 parts by massof the carbon black N220, 125 parts by mass of the silica-1 as thesilica (B), 9.6 parts by mass of the silane coupling agent Si75 as thesilane coupling agent (C) and 50 parts by mass of an aromatic oil werekneaded for 60 seconds, and then 1 part by mass of1,3-diphenylguanidine, which is a guanidine compound, as thevulcanization accelerator (D) was added and further kneaded, in whichthe maximum temperature of the rubber composition in the first step ofkneading was regulated to 150° C.

Subsequently, in the final step of kneading, 2 parts by mass of stearicacid, 1 part by mass of the anti-aging agent 6PPD, 1 part by mass of theanti-aging agent TMDQ, 2.5 parts by mass of zinc flower, 1.2 parts bymass of 1,3-diphenylguanidine, 1.2 part by mass of the vulcanizationaccelerator MBTS, 0.7 part by mass of the vulcanization accelerator TBBSand 1.7 parts by mass of sulfur were added, in which the maximumtemperature of the rubber composition in the final step of kneading wasregulated to 110° C.

The vulcanized rubber composition obtained from the rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 6.

Example 22

The kneading operation was performed in the same manner as in Example21, except that 50 parts by mass of the emulsion-polymerized SBR-1 and50 parts by mass of the solution-polymerized SBR-2 were used as therubber component (A). The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 6.

Example 23

The kneading operation was performed in the same manner as in Example21, except that 67 parts by mass of the emulsion-polymerized SBR-1 and33 parts by mass of the solution-polymerized SBR-2 were used as therubber component (A). The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 6.

Example 24

The kneading operation was performed in the same manner as in Example21, except that 100 parts by mass of the emulsion-polymerized SBR-1 wasused as the rubber component (A), and the maximum temperature of therubber composition in the first step of kneading was regulated to 170°C. The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 6.

Comparative Example 25

The kneading operation was performed in the same manner as in Example21, except that 1 part by mass of 1,3-diphenylguanidine was not added inthe first step of kneading, neither 2 parts by mass of stearic acid nor1 part by mass of the anti-aging agent 6PPD was added in the final stepof kneading, and 100 parts by mass of the solution-polymerized SBR-2 wasused as the rubber component (A), and 2 parts by mass of stearic acidand 1 part by mass of the anti-aging agent 6PPD were added, in the firststep of kneading. The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 6.

Comparative Example 26

The kneading operation was performed in the same manner as inComparative Example 25, except that 40 parts by mass of theemulsion-polymerized SBR-1 and 60 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 6.

Comparative Example 27

The kneading operation was performed in the same manner as inComparative Example 25, except that 50 parts by mass of theemulsion-polymerized SBR-1 and 50 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 6.

Comparative Example 28

The kneading operation was performed in the same manner as inComparative Example 25, except that 67 parts by mass of theemulsion-polymerized SBR-1 and 33 parts by mass of thesolution-polymerized SBR-2 were used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 6.

Comparative Example 29

The kneading operation was performed in the same manner as inComparative Example 25, except that 100 parts by mass of theemulsion-polymerized SBR-1 was used as the rubber component (A). Theresulting vulcanized rubber composition was evaluated for the averageaggregated aggregate area and the low-heat-generation property (tan δindex) according to the aforementioned manners. The results are shown inTable 6.

TABLE 3 Example Comparative Example 9 10 11 12 10 11 12 13 14 Averageaggregated aggregate 1480 1520 1530 1530 1940 1940 1940 1940 1940 areaof vulcanized rubber composition (nm²) Vulcanization property: 108 106105 104 100 100 100 100 100 low-heat-generation property (tanδ index)

TABLE 4 Example Comparative Example 13 14 15 16 15 16 17 18 19 Averageaggregated aggregate 1670 1700 1700 1710 2080 2080 2080 2080 2080 areaof vulcanized rubber composition (nm²) Vulcanization property: 110 107106 105 100 100 100 100 100 low-heat-generation property (tanδ index)

TABLE 5 Example Comparative Example 17 18 19 20 20 21 22 23 24 Averageaggregated aggregate 1730 1760 1770 1770 2170 2170 2170 2170 2170 areaof vulcanized rubber composition (nm²) Vulcanization property: 109 106105 105 100 100 100 100 100 low-heat-generation property (tanδ index)

TABLE 6 Example Comparative Example 21 22 23 24 25 26 27 28 29 Averageaggregated aggregate 1800 1840 1850 1850 2250 2250 2250 2250 2250 areaof vulcanized rubber composition (nm²) Vulcanization property: 109 106105 105 100 100 100 100 100 low-heat-generation property (tanδ index)

Example 25

The kneading operation was performed in the same manner as in Example 9,except that 25 parts by mass of the natural rubber and 75 parts by massof the solution-polymerized SBR-2 were used as the rubber component (A).The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 7.

Example 26

The kneading operation was performed in the same manner as in Example 9,except that 50 parts by mass of the natural rubber and 50 parts by massof the solution-polymerized SBR-2 were used as the rubber component (A).The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 7.

Example 27

The kneading operation was performed in the same manner as in Example 9,except that 67 parts by mass of the natural rubber and 33 parts by massof the solution-polymerized SBR-2 were used as the rubber component (A).The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 7.

Example 28

The kneading operation was performed in the same manner as in Example 9,except that 100 parts by mass of the natural rubber was used as therubber component (A). The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 7.

Comparative Example 30

The kneading operation was performed in the same manner as inComparative Example 10, except that 25 parts by mass of the naturalrubber and 75 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 7.

Comparative Example 31

The kneading operation was performed in the same manner as inComparative Example 10, except that 50 parts by mass of the naturalrubber and 50 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 7.

Comparative Example 32

The kneading operation was performed in the same manner as inComparative Example 10, except that 67 parts by mass of the naturalrubber and 33 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 7.

Comparative Example 33

The kneading operation was performed in the same manner as inComparative Example 10, except that 100 parts by mass of the naturalrubber was used as the rubber component (A). The resulting vulcanizedrubber composition was evaluated for the average aggregated aggregatearea and the low-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 7.

Example 29

The kneading operation was performed in the same manner as in Example13, except that 25 parts by mass of the natural rubber and 75 parts bymass of the solution-polymerized SBR-2 were used as the rubber component(A). The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 8.

Example 30

The kneading operation was performed in the same manner as in Example13, except that 50 parts by mass of the natural rubber and 50 parts bymass of the solution-polymerized SBR-2 were used as the rubber component(A). The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 8.

Example 31

The kneading operation was performed in the same manner as in Example13, except that 67 parts by mass of the natural rubber and 33 parts bymass of the solution-polymerized SBR-2 were used as the rubber component(A). The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 8.

Example 32

The kneading operation was performed in the same manner as in Example13, except that 100 parts by mass of the natural rubber was used as therubber component (A). The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 8.

Comparative Example 34

The kneading operation was performed in the same manner as inComparative Example 15, except that 25 parts by mass of the naturalrubber and 75 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 8.

Comparative Example 35

The kneading operation was performed in the same manner as inComparative Example 15, except that 50 parts by mass of the naturalrubber and 50 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 8.

Comparative Example 36

The kneading operation was performed in the same manner as inComparative Example 15, except that 67 parts by mass of the naturalrubber and 33 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 8.

Comparative Example 37

The kneading operation was performed in the same manner as inComparative Example 15, except that 100 parts by mass of the naturalrubber was used as the rubber component (A). The resulting vulcanizedrubber composition was evaluated for the average aggregated aggregatearea and the low-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 8.

Example 33

The kneading operation was performed in the same manner as in Example17, except that 25 parts by mass of the natural rubber and 75 parts bymass of the solution-polymerized SBR-2 were used as the rubber component(A). The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 9.

Example 34

The kneading operation was performed in the same manner as in Example17, except that 50 parts by mass of the natural rubber and 50 parts bymass of the solution-polymerized SBR-2 were used as the rubber component(A). The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 9.

Example 35

The kneading operation was performed in the same manner as in Example17, except that 67 parts by mass of the natural rubber and 33 parts bymass of the solution-polymerized SBR-2 were used as the rubber component(A). The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 9.

Example 36

The kneading operation was performed in the same manner as in Example17, except that 100 parts by mass of the natural rubber was used as therubber component (A). The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 9.

Comparative Example 38

The kneading operation was performed in the same manner as inComparative Example 20, except that 25 parts by mass of the naturalrubber and 75 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 9.

Comparative Example 39

The kneading operation was performed in the same manner as inComparative Example 20, except that 50 parts by mass of the naturalrubber and 50 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 9.

Comparative Example 40

The kneading operation was performed in the same manner as inComparative Example 20, except that 67 parts by mass of the naturalrubber and 33 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 9.

Comparative Example 41

The kneading operation was performed in the same manner as inComparative Example 20, except that 100 parts by mass of the naturalrubber was used as the rubber component (A). The resulting vulcanizedrubber composition was evaluated for the average aggregated aggregatearea and the low-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 9.

Example 37

The kneading operation was performed in the same manner as in Example21, except that 25 parts by mass of the natural rubber and 75 parts bymass of the solution-polymerized SBR-2 were used as the rubber component(A). The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 10.

Example 38

The kneading operation was performed in the same manner as in Example21, except that 50 parts by mass of the natural rubber and 50 parts bymass of the solution-polymerized SBR-2 were used as the rubber component(A). The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 10.

Example 39

The kneading operation was performed in the same manner as in Example21, except that 67 parts by mass of the natural rubber and 33 parts bymass of the solution-polymerized SBR-2 were used as the rubber component(A). The resulting vulcanized rubber composition was evaluated for theaverage aggregated aggregate area and the low-heat-generation property(tan δ index) according to the aforementioned manners. The results areshown in Table 10.

Example 40

The kneading operation was performed in the same manner as in Example21, except that 100 parts by mass of the natural rubber was used as therubber component (A). The resulting vulcanized rubber composition wasevaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 10.

Comparative Example 42

The kneading operation was performed in the same manner as inComparative Example 25, except that 25 parts by mass of the naturalrubber and 75 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 10.

Comparative Example 43

The kneading operation was performed in the same manner as inComparative Example 25, except that 50 parts by mass of the naturalrubber and 50 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 10.

Comparative Example 44

The kneading operation was performed in the same manner as inComparative Example 25, except that 67 parts by mass of the naturalrubber and 33 parts by mass of the solution-polymerized SBR-2 were usedas the rubber component (A). The resulting vulcanized rubber compositionwas evaluated for the average aggregated aggregate area and thelow-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 10.

Comparative Example 45

The kneading operation was performed in the same manner as inComparative Example 25, except that 100 parts by mass of the naturalrubber was used as the rubber component (A). The resulting vulcanizedrubber composition was evaluated for the average aggregated aggregatearea and the low-heat-generation property (tan δ index) according to theaforementioned manners. The results are shown in Table 10.

TABLE 7 Example Comparative Example 25 26 27 28 30 31 32 33 Averageaggregated aggregate area of 1520 1540 1550 1550 1940 1940 1940 1940vulcanized rubber composition (nm²) Vulcanization property: 108 106 104104 100 100 100 100 low-heat-generation property (tanδ index)

TABLE 8 Example Comparative Example 29 30 31 32 34 35 36 37 Averageaggregated aggregate area 1670 1700 1710 1710 2100 2100 2100 2100 ofvulcanized rubber composition (nm²) Vulcanization property: 110 108 107106 100 100 100 100 low-heat-generation property (tanδ index)

TABLE 9 Example Comparative Example 33 34 35 36 38 39 40 41 Averageaggregated aggregate area 1720 1760 1770 1770 2160 2160 2160 2160 ofvulcanized rubber composition (nm²) Vulcanization property: 108 107 106105 100 100 100 100 low-heat-generation property (tanδ index)

TABLE 10 Example Comparative Example 37 38 39 40 42 43 44 45 Averageaggregated aggregate area 1780 1810 1810 1810 2220 2220 2220 2220 ofvulcanized rubber composition (nm²) Vulcanization property: 107 106 105104 100 100 100 100 low-heat-generation property (tanδ index)

As apparent from Tables 1 and 2, the rubber compositions of Examples 1to 8 have good low-heat-generation property (tan δ index) as compared tothe rubber compositions of Comparative Examples 1 to 9.

As apparent from Tables 3 and 10, the rubber compositions of Examples 9to 40 have good low-heat-generation property (tan δ index) as comparedto the rubber compositions of Comparative Examples 10 to 45.

INDUSTRIAL APPLICABILITY

The rubber composition of the present invention is excellent inlow-heat-generation property, and thus is favorably used as members ofpneumatic tires for a passenger car, a pickup truck, a light passengercar, a light truck and a heavy vehicle (such as a truck, a bus and aconstruction vehicle), and particularly a tread member of a pneumaticradial tire.

The invention claimed is:
 1. A rubber composition comprising: (A) arubber component containing 10% by mass or more of at least one kind ofrubber selected from diene rubber synthesized by emulsion polymerizationand natural rubber and 90% by mass or less of another kind of dienerubber; (B) a silica having a n-hexadecyltrimethylammonium bromide(CTAB) adsorption specific surface area of not less than 180 m²/gmeasured according to a method described in ASTM D3765-92; (C) at leastone silane coupling agent selected from a polysulfide compound and athioester compound; and (D) a vulcanization accelerator, the rubbercomposition after vulcanization having an average aggregated aggregatearea (nm²) of the silica of 1,900 or less, measurement method of averageaggregated aggregate area: an upper surface of a specimen of the rubbercomposition after vulcanization is cut in a direction making an angle of38° with respect to the upper surface of the specimen with a focused ionbeam; then a smooth surface of the specimen formed by cutting is imagedwith a scanning electron microscope at an acceleration voltage of 5 kVin a direction perpendicular to the smooth surface; the resulting imageis converted to a binarized image of a rubber portion and a silicaportion as a filler of the specimen by the Otsu's method; an aggregatedaggregate area of the silica portion is obtained based on the resultingbinarized image; and the average aggregated aggregate area of the silicaportion is calculated in terms of number average (arithmetic average)per unit area (3 μm×3 μm) from a total surface area of the silicaportion and the number of aggregated aggregates, provided that in thecalculation, a particle that is in contact with an edge of the image isnot counted, and a particle of 20 pixels or less is assumed to be noiseand is not counted.
 2. The rubber composition according to claim 1,wherein the another kind of diene rubber is at least one kind of rubberselected from solution-polymerized styrene-butadiene copolymer rubber,polybutadiene rubber and synthesized polysoprene rubber.
 3. The rubbercomposition according to claim 1, wherein the silane coupling agent (C)is at least one compound selected from the compounds represented by thefollowing general formulae (I) to (IV): [Chem. 1](R¹O)_(3-p)(R²)_(p)Si—R³—S_(a)—R³—Si(OR¹)_(3-r)(R²)_(r)  (I) wherein R¹,which may be the same or different, each represents a linear, cyclic orbranched alkyl group, having from 1 to 8 carbon atoms, or a linear orbranched alkoxylalkyl group, having from 2 to 8 carbon atoms; R², whichmay be the same or different, each represents a linear, cyclic orbranched alkyl group, having from 1 to 8 carbon atoms; R³, which may bethe same or different, each represents a linear or branched alkylenegroup, having from 1 to 8 carbon atoms; a indicates from 2 to 6 as amean value; p and r, which may be the same or different, each indicatefrom 0 to 3 as a mean value, provided that both p and r are not 3 at thesame time,

wherein R⁴ represents a monovalent group selected from —Cl, —Br, R⁹O—,R⁹C(═O)O—, R⁹R¹⁰C═NO—, R⁹R¹⁰CNO—, R⁹R¹⁰N—, and—(OSiR⁹R¹⁰)_(h)(OSiR⁹R¹⁰R¹¹) (where R⁹, R¹⁰ and R¹¹, which may be thesame or different, each represent a hydrogen atom or a monovalenthydrocarbon group having from 1 to 18 carbon atoms; and h indicates from1 to 4 as a mean value); R⁵ represents R⁴, a hydrogen atom, or amonovalent hydrocarbon group having from 1 to 18 carbon atoms; R⁶represents R⁴, R⁵, a hydrogen atom, or a group —[O(R¹²O)_(j)]_(0.5)(where R¹² represents an alkylene group having from 1 to 18 carbonatoms; and j indicates an integer of from 1 to 4); R⁷ represents adivalent hydrocarbon group having from 1 to 18 carbon atoms; R⁸represents a monovalent hydrocarbon group having from 1 to 18 carbonatoms; x, y and z each indicate a number satisfying the relationship ofx+y+2z=3, 0≦x≦3, 0≦y≦2, 0≦z≦1,[Chem. 3](R¹³O)_(3-s)(R¹⁴)_(s)Si—R¹⁵—S_(k)—R¹⁶—S_(k)—R¹⁵—Si(OR¹³)_(3-t)(R¹⁴)_(t)  (III)wherein R¹³, which may be the same or different, each represents alinear, cyclic or branched alkyl group, having from 1 to 8 carbon atomsor a linear or branched alkoxylalkyl group, having from 2 to 8 carbonatoms; R¹⁴, which may be the same or different, each represents alinear, cyclic or branched alkyl group, having from 1 to 8 carbon atoms;R¹⁵, which may be the same or different, each represents a linear orbranched alkylene group, having from 1 to 8 carbon atoms; R¹⁶ representsa divalent group of a general formula (—S—R¹⁷—S—), (—R¹⁸—S_(m1)—R¹⁹—) or(—R²⁰—S_(m2)—R²¹—S_(m3)—R²²—) (where R¹⁷ to R²², which may be the sameor different, each represent a divalent hydrocarbon group, a divalentaromatic group or a divalent organic group containing a hetero elementexcept sulfur and oxygen, having from 1 to 20 carbon atoms; m1, m2 andm3 may be the same or different, each indicating from 1 to less than 4as a mean value); k, which may be the same or different, each indicatefrom 1 to 6 as a mean value; s and t, which may be the same ordifferent, each indicate from 0 to 3 as a mean value, provided that boths and t are not 3 at the same time,

wherein R²³ represents a linear, branched or cyclic alkyl group, havingfrom 1 to 20 carbon atoms; G, which may be the same or different, eachrepresent an alkanediyl group or an alkenediyl group, having from 1 to 9carbon atoms; Z^(a), which may be the same or different, each representa group capable of bonding to the two silicon atoms and selected from[—O—]_(0.5), [—O-G-]_(0.5) and [—O-G-O—]_(0.5); Z^(b), which may be thesame or different, each represent a group which is capable of bonding tothe two silicon atoms and is the functional group represented by[—O-G-O—]_(0.5); Z^(c), which may be the same or different, eachrepresent a functional group selected from —Cl, —Br, —OR^(a),R^(a)C(═O)O—, R^(a)R^(b)C═NO—, R^(a)R^(b)N—, R^(a)— and HO-G-O— (where Gis the same as above); R^(a) and R^(b), which may be the same ordifferent, each represent a linear, branched or cyclic alkyl group,having from 1 to 20 carbon atoms; m, n, u, v and w, which may be thesame or different, each are 1≦m≦20, 0≦n≦20, 0≦u≦3, 0≦v≦2, 0≦w≦1, and(u/2)+v+2w=2 or 3; in case where the formula has multiple A's, thenZ^(a) _(u), Z^(b) _(v) and Z^(c) _(w) may be the same or different inthose multiple A's; in case where the formula has multiple B's, thenZ^(a) _(u), Z^(b) _(v) and Z^(c) _(w) may be the same or different inthose multiple B's.
 4. The rubber composition according to claim 3,wherein the silane coupling agent (C) is a compound represented by thegeneral formula (I).
 5. The rubber composition according to claim 1,wherein the rubber composition contains the silica (B) in an amount offrom 25 to 150 parts by mass per 100 parts by mass of the rubbercomponent (A).
 6. The rubber composition according to claim 1, whereinthe silica (B) is precipitation method silica.
 7. The rubber compositionaccording to claim 1, wherein the rubber composition further comprisescarbon black.